8hp Sailmaster wont rev in gear

[Phylarch]

I have an 1986 Johnson 8hp Sailmaster longshaft attached to a Chrysler 22ft sailboat. boat weighs aprox 3000lb. The engine runs great in a trashcan, revs up in forward , reverse and neutral. When attached to the boat , the engine will not rev up in gear. It will rev up in neutral, but once in gear in will not increase in rpm and move boat. It will idle in gear, and move the boat very slowly, but not enough power to overcome a light wind. The compression is 125psi top and bottom, I'm running 50:1, Ive run the engine in a trash can for more than 30 minutes, no over heating and no fuel flow problems. While attached to the boat, the engine sits about 18-20 inches in the water. ( I thought it may be too low in water, but seems to match pictures of other boats same size and weight) Any suggestions would be appreciated.

 

[racerone]

Sounds like it is running on one cylinder.-----Check for spark on both leads.

 

[Vic.S]

As suggested check for two good sparks and/or do a cylinder drop test.

Compressions look good.

The Sailmaster engines are configured specifically for use on a sail boat.. They are designed to set the prop lower in the water to which end they have a modified exhaust outlet so that the exhaust back pressure is not increased. They also have a high thrust prop.

I have a 6hp Evinrude Yachtwin engine on my 19ft sail boat

Mine has a 20" shaft length but is mounted so that the extra 5" over the standard 15" shaft models puts the prop 5" deeper. I beleive the 8Hp models have a 25" shaft and should be mounted so that the prop is 5" deeper than standard models and the power head is 5" higher.. This means that the upper of the two AV plates should be about 3 to 5" below the water surface.

50:1 is the correct fuel mix even if the decals and owners manual say 100:1

 

[Phylarch]

Ok so I did a cylinder drop test. (I'm assuming this is done by pulling a plug wire while it's running) When I pulled each plug wire, the motor would continue to run, but very rough on one cylinder. So I feel like it's running on both cylinders and both are getting good spark. My problem only occurs when the motor is under a load. It runs fine in the tank, revs up in forward and reverse, and of course neutral, but when attached to the sailboat in the lake it just bogs down in gear. It'll run for about 30 seconds then die. I'm baffled!

 

[oldboat1]

Fuel tank location in the boat might be the difference (vs. testing in a barrel) -- pump might not be up to it.

 

[Vic.S]

Ok so I did a cylinder drop test. (I'm assuming this is done by pulling a plug wire while it's running) When I pulled each plug wire, the motor would continue to run, but very rough on one cylinder. So I feel like it's running on both cylinders and both are getting good spark. My problem only occurs when the motor is under a load. It runs fine in the tank, revs up in forward and reverse, and of course neutral, but when attached to the sailboat in the lake it just bogs down in gear. It'll run for about 30 seconds then die. I'm baffled!

I reckon probably a fuel flow or carb problem. It does not need much fuel or a very wide throttle opening to run at full rpm in neutral.
I assume it revs OK in neutral on the boat.and that the tank vent is open and clear

If its a pump problem or air being drawn in ( eg through a dodgy fuel line connector) continually squeezing the bulb will keep it running, but if that does no good then maybe the carb is in need of cleaning

 

[racerone]

When you did the cylinder drop test it ran smoothly on one cylinder.----Not the other one.----------Well that clearly indicates that something is wrong with the cylinder that was running rough.-----Time to investigate the problem !!-----Perhaps as simple as a bad diaphragm ( $4.00 item ) in the fuel pump.---And It runs fine in a tank because lots of exhaust bubbles do affect the load on the prop.-----Does the primer bulb go hard when you operate it ??

 

[Phylarch]

Fuel tank location in the boat might be the difference (vs. testing in a barrel) -- pump might not be up to it.

The motor is attached to the boat, I just put motor in a large garbage can, and fill it to the top. prop is about 10 inches under surface. No change in tank height or location.

VicS. - It revs to WOT in neutral, AND in gear in tank. Forward and reverse throw all the water out of tank at WOT!! It just wont rev in gear when boat is in lake (under a load) It will rev in neutral when boat is in lake. Could be a dodgy fuel line connection.

Racerone - Sorry for my poor sentence structure. I should be more clear. It runs rough on EACH cylinder when opposite plug wire pulled. Same result of rough idle when either wire pulled. Primer bulb is very firm when motor is running, I wouldn't say hard.

Thanks for all the great advice. I think my next move is to replace fuel line, bulb and fuel line connections. I don't know how old they are or if they're leaking air. Thanks again guys.

 

[racerone]

Good luck with the project.